It is well known that wherever possible, 3 phase, squirrel cage induction motors are preferred for driving the load. However, when the load requires variable speed, in early days, DC motors were invariably deployed. While being easily amenable to speed control by simple means, the DC motors with the Commutator/Carbon Brush Gear are less reliable and prone to higher maintenance. Hence there has always been a concerted effort to find a simple solution for the speed control of the AC induction motor.
However the complexity of the speed control of the AC motor has been the major drawback vis-a-vis the DC motor. In a DC motor the speed control is very simple in that it is achieved by simply varying the voltage applied to the armature. But in induction motors, both the voltage and frequency have to be varied simultaneously. Also during the power conversion stage it is necessary to ensure that the output waveforms are as near to sinusoid as possible to minimize harmonic effects and reduce losses, noise and vibration. It has often been the goal of the designer to come out with a simple and cost-effective design of the power electronics and the Pulse Width Modulation (PWM) controller to achieve the variability of speed in the three-phase induction motor.
In the recent past there has been increasing efforts to devise ways and means of the variable voltage-variable frequency method of speed control of induction motors. This is mainly because the three phase induction motor is perhaps the most rugged and reliable rotating machine that forms the workhorse of the industry. Its die-cast cage rotor is virtually indestructible and the absence of commutator and brush gear makes these motors, the most widely used in the industry. Wherever possible and where a three-phase supply is available, the three-phase motor is preferred to the single-phase induction motor as well. The three-phase motor is also having the least weight and size for a given horsepower at a particular speed. The single-phase induction motor is generally less efficient than a three phase one and also has additional components like starting switch and capacitor. They bring down the reliability of the machine.
To vary the speed of the induction motor, both the voltage and frequency need to be varied in tandem below the rated speed of the motor. For speeds above rated value, only the frequency is varied while the voltage is kept constant. The general practice is to rectify and filter the input AC supply to DC and invert the same to variable voltage and variable frequency AC. While doing this it is preferable to have the output current waveform as near sinusoidal as possible. This is because the induction motor operates best with sinusoidal magnetic flux.
There have been many approaches to obtain such a sinusoidal variable voltage-variable frequency output from the inverter. Most of the earlier methods used analog circuitry with a lot of hardware to obtain the PWM wave generation. These circuits employ triangular carrier wave at a higher frequency and different sampling techniques to obtain the pulse width modulated output wave. Such analog circuits invariably have limitations in that they are complex and expensive. They are also prone to drift due to aging of the components and thermal run-away due to heating. Frequent factory and field adjustments of the circuitry are required. Also the design gets frozen once the product is made and even a minor change in the design later would necessitate another round of prototyping and fabrication. Also such hardware intensive circuitry always had the associated reliability problems and high costs.
Subsequently, in recent times there have been attempts to arrive at the solution to obtain the pulse width modulated, sinusoidal variable voltage-variable frequency output by means of employing microprocessor based controllers. Such an approach is engaging the attention of the contemporary designers as may be seen from U.S. Pat. Nos. 4,636,928, 4,599,550, 4,656,572, 5,140,248, 5,495,160, based on this approach.
The earlier approach in this method is to digitally store the waveform of the voltage in the read only memory (ROM) of the microcontroller and read the same at appropriate intervals by means of interrupts. The program suitably handles the interrupts and the output of the processor to send the switching signals to the inverter.
It was also suggested that the sine values during the entire 360.degree. of one cycle of the waveform are stored in the form of a look up table and the program read the same at the regular intervals and appropriately switches the Inverter Bridge. In both these approaches the memory requirement of the program is generally large, of the order of 4K or more bytes. This is mainly because of the memory requirement of the look-up table as well as the main code itself. Also once the code is larger, it becomes necessary to deploy faster processors with lower instruction cycle times to effect the control in real-time and such hardware add to the cost.
In another case a mathematical approach was adopted to digitally generate the sinusoidal wave. In this a mathematical algorithm based on the Bresenham technique was used to synthesize a circle and two waves corresponding to the x and y coordinates are generated to follow the contour of this reference circle. These digital sine waves are then converted by means of a 2/3-phase converter and modulator to obtain the three phase, pulse width modulated signals. Here also equipment employs quite a bit of hardware in the form of Timers, Counters, Frequency units, Multiplexers and other logic units which makes the apparatus quite expensive to be deployed in price sensitive applications.
There are several PWM techniques as described by J. Holz in his research paper, "Pulse Width Modulation--A Survey", IEEE Transactions Industrial Electronics, vol. 39, no. 5, pp. 410-420, 1992. The principle and the methodology involved are described in greater details in publications given below:
R. M. Park, "Two--reaction Theory of Synchronous Machines, Part. I, Generalized Method of Analysis", AIEE Trans., vol. 48, no. 1, pp. 716-730, July 1929.
T.G. Habetler, "A Space Vector-Based Rectifier Regulator for AC/DC/AC Converters", IEEE Trans. Power Electronics, vol. 8, no. 1, pp. 30-36, 1993.
One of the objects of the invention is to obviate the above drawbacks by varying the magnitude and frequency of the applied voltage, while keeping the output waveform of the inverter as close to sinusoidal as required by using a micro-processor based controller, which is programmable to achieve the switching configuration as required by SVPWM or Sinusoidal PWM (SPWM) technique.
Another object of this invention is to keep the code length and the memory requirement of the microprocessor at a minimum level so that SVPWM or SPWM can be implemented at a low cost and the controller can be used in appliance motors, high frequency tools and industrial equipment.